1. Field of the Invention
The invention pertains generally to demodulation circuits for transducers and is more particularly directed to compensation circuitry for a quad-diode demodulator and capacitive transducer combination.
2. Prior Art
Capacitive transducers are useful devices for measuring physical parameters such as pressure, distance, surface roughness, angle change or the like and come in an almost limitless variety of shapes, sizes and configurations. A change in the sensed parameter will cause the transducer to vary its capacitance accordingly in a proportional or known functional manner. The change in capacitance of the transducer can thereafter be utilized in a number of ways to generate an electrical signal representative of the change in the physical parameter.
Generally, for capacitive transducers, the generation of the electrical signal is accomplished by the modulation of an alternating carrier frequency where one of the characteristics of the carrier is modified electrically by the variable capacitance of the transducer. The carrier is thereafter detected or demodulated to obtain the intelligence contained and thereby generates a useful electrical signal representative of the sensed parameter. The circuit used for modulating the carrier is usually in integral combination with the demodulator circuit with the whole being termed hereinafter a transducer demodulator.
Particularly, such a transducer demodulator circuit configuration using a quad-diode is shown in U.S. Pat. Nos. 3,318,153; 3,271,669 issued to T. Lode. These circuits are particularly useful for capacitive pressure transducers. Another advantageous type of capacitive transducer demodulator that has been recently developed is the quad-diode bridge circuit. An example of which is illustrated in U.S. Pat. Nos. 3,883,812; 3,869,676 issued to Harrison et al. The desirable characteristics of this demodulator include a sufficient magnitude of output which is relatively independent of excitation waveform and frequency. Additionally, this transducer demodulator provides excellent resolution for the change in capacitance of the transducer and allows the transducer to be conveniently grounded. These are features that will provide for greater use of this circuit in multi-farious transducer applications in the future. These circuits and others of their general type will herein further be termed quad-diode demodulators because of their circuitry utilizing four rectification devices or diodes.
If the capacitive transducer is a pressure transducer, a capacitive transducer and demodulator combination, as described above, can be utilized for sensing manifold absolute pressure (MAP) changes in an internal combustion engine. The electrical signal obtained from the combination can then be used as is conventional to regulate functional aspects of the engine operation such as air/fuel ratio, timing, EGR, etc. when sensed with other engine parameters. In the automotive environment the convenient grounding of a capacitive transducer is an important feature which allows a direct connection to the chassis and eliminates the problems of isolating a transducer with a reference potential that is above or below chassis ground. Also, the referenced Harrison quad-diode circuit lends itself to remote transducer applications which probably will accompany many new developments in automotive electronics.
However, there are still problems with using these quad-diode demodulators in harsh environments such as that found in the engine compartment of an automobile. The range of temperatures through which the transducer-demodulator circuitry is subjected to is extreme (40.degree. F. to 120.degree. F.) and the circuitry must, therefore, be provided with accurate temperature compensation. This problem is complicated by the inclusion of the necessary but non-linear diodes in the demodulator circuitry. These diodes will produce not only different voltage drops for different temperatures but will also produce different voltage drops at the same temperature when different currents are conducted.
Another problem found in many environments but which is particularly troublesome in the automotive environment is the regulation of the power supply. With constantly changing demands on a limited battery and only a rough regulation from the voltage regulator for alternator voltage changes, surges and voltage drops of significant magnitude are not uncommon. Transducer electronics where the information is contained within the amplitude of transducer signal and changes with respect to a reference are particularly affected by these changes.
One method developed for overcoming this problem is ratiometry. This method contemplates that the output of a particular circuit will change in accordance with the changes in the power supply to always remain a predetermined percentage of the power supply for non-signal conditions. Thus, when a plurality of these circuits are connected together signal information will not be lost and errors will not be introduced because of the regulation problems of the power supply. Therefore, when operated in an automotive or other environments where regulation problems are prevalent, the quad-diode demodulator and capacitive transducer combination should be provided with ratiometric compensation for facile connection to other system circuitry. Compensation for ratiometric errors is difficult because of the non-linear nature of the diodes of the demodulator which cause an error.
In certain instances it is just as important to compensate for the transducer itself as it is to compensate the demodulator circuitry. For example, quartz capacitor transducers are relatively accurate and inexpensive but they are temperature sensitive and some have linearity problems for reasonably priced transducers. It would be extremely advantageous to compensate a low cost quartz capacitive transducer to provide a linear output without temperature dependency while retaining the desirable features of a quad-diode demodulator.